1. Field of the Invention
The present invention relates to a fuel cell system and a control method thereof.
2. Description of Related Art
Japanese Patent Application Publication No. 2012-004138 (JP 2012-004138 A) describes a fuel cell system configured such that: when it is determined that a fuel cell is in a transient operation state, the fuel cell system performs a control to increase a pressure target value of an oxidant gas to be larger than a pressure target value for a normal operation, so as to prevent abnormal decrease of an output voltage of the fuel cell when abnormality occurs in an air supply system. Published Japanese Patent Application Publication No. 2012-109182 (JP-A-2012-109182) describes a fuel cell system configured such that: in a case of a transient decrease in which a load decreases, when a detected dry/wet state of a fuel cell is an excessively dry state or an excessively wet state, a flow rate of a cathode gas or a decrease rate of a pressure is controlled according to the state thus detected so as to moderate the state, thereby controlling an amount of water to be carried away by the cathode gas, such that the dry/wet state of the fuel cell is controlled sufficiently. International Publication No. 2012/117937 describes a fuel cell system configured such that: at the time of a transient operation in which an operation state changes, a wet state of an electrolyte membrane is controlled so as to gradually change from a wet state, which is detected before the transient operation starts, to a steady time target wet state based on a set transient time target wet state, thereby preventing a transitional large change of a flow rate or the like of a reaction gas from deteriorating fuel economy and sound vibration performance.
However, in JP 2012-004138 A, depending on an operation state after the transient operation, the wet state of the cell may be further deteriorated. For example, in a case where a request output decreases in a state of a further wet side relative to an appropriate wet state of the fuel cell, there is such a possibility that the wet state of the cell becomes an excessively wet state. Further, when a request output increases in a state on a dry side, there is such a possibility that the wet state of the cell becomes an excessively dry state. Accordingly, there is such a possibility that an available output of the fuel cell is limited depending on an operation state after the transient operation, and the fuel cell cannot output the request output. Further, as described above, in JP-A-2012-109182, the amount of water to be carried away by the cathode gas can be controlled according to a detected state, such that the dry/wet state of the fuel cell can be controlled. Moreover, as described above, in International Publication No. 2012/117937, at the time of the transient operation in which the operation state changes, the wet state of the electrolyte membrane is controlled to gradually change from the wet state detected before the transient operation starts to the steady time target wet state, based on the set transient time target wet state. With such a configuration, similarly to the case of JP-A-2012-109182, the dry/wet state of the fuel cell can be controlled. However, the control of the dry/wet state of the fuel cell by the techniques of JP-A-2012-109182 and International Publication No. 2012/117937 is complicated, so it has been desired to realize the control by a simple and easy technique.